Soft devices are machines built from soft materials (e.g. elastomers, gels, liquids). These soft devices are useful for their ability to change their size and shape readily upon electrical, chemical, pneumatic, ferrofluidic, or hydraulic actuation. In addition, the low stiffness of the materials used to construct these devices (Young's modulus <10 MPa) enables them to deform readily in response to external forces. These attributes allow soft devices to perform functions that are challenging for hard machines. Examples include interacting with delicate, soft materials (e.g. biological tissues), and performing unstructured tasks (e.g. gripping objects of undefined shape).
Integrating electronics for control systems and sensors into soft devices will be an important step in their evolution. However, metal wires, used in conventional electronics, when embedded in a soft device, often delaminate from the surrounding soft materials, or break, when the device bends and stretches—as it would during operation.
In recent years, a wide variety of soft embodiments of common electronic devices (e.g. transistors, displays, batteries, electromechanical transducers, speakers, thermal sensors, strain sensors, pressure sensors, and photo detectors) have been successfully fabricated. To meet the electrical needs of these devices, an array of stretchable electrical conduits for supplying potential and current have been created. For example, liquid metals have been used to create stretchable wires by embedding channels filled with EGaIn into elastomers to achieve highly conductive structures. Metal ion implantation can also be used to create stretchable conductive structures requiring either the use of a plasma chamber or multiple, wet-chemistry, processing steps. One mechanical approach is to create conductive, net-shaped, structures out of relatively inelastic materials using mechanical processing with controlled cutting tools to convert ridged substrates for electrical components (e.g. polyimide sheets) into stretchable nets, although they have also been fabricated via molding of PDMS. Recently, hydrogels infused with ionic conductors have been used to create transparent, biocompatible stretchable devices; but these materials do not tolerate continuous DC currents. One of the oldest and most thoroughly investigated approaches to making stretchable conductors is to create conductive particle composites that rely on a percolation network of conductive particles to conduct electricity. Though these materials are not appropriate replacements for conventional wires since their resistivity is typically large and can change by several orders of magnitude during reorganization of the percolation network induced by material strain. To date, the most successful approaches to creating small, stretchable wires has used lithographic techniques to create serpentine patterns of metal films on unstrained elastomers or on pre-strained elastomers to make buckled metal films. These approaches have enabled the creation of wires capable of sustaining high strains (typically >30% with strains >300% having been reported for serpentine patterns).